4.1.5. Errors caused by selection bias

Selection bias should also be considered as a source of error. Selecting
an unrepresentative sample will lead to bias in the results. Actively selecting
a random sample rather than a self-selecting sample who may well differ from
those not included in the survey (volunteer bias), is the best way to avoid
this. Using well-trained personnel will avoid any haphazard substitution of
properly selected participants with more conveniently available participants
who had not been selected, which is another possible source of selection bias.